Design Tradeoffs and Challenges in Practical Coherent Optical Transceiver Implementations

Abstract

This tutorial discusses the design and ASIC implementation of coherent optical transceivers. Algorithmic and architectural options and tradeoffs between performance and complexity/power dissipation are presented. Particular emphasis is placed on flexible (or reconfigurable) transceivers because of their importance as building blocks of software defined optical networks. The article elaborates on some advanced digital signal processing (DSP) techniques such as iterative decoding, which are likely to be applied in future coherent transceivers based on higher order modulations. Complexity and performance of critical DSP blocks such as the forward error correction decoder and the frequency-domain bulk chromatic dispersion equalizer are analyzed in detail. Other important ASIC implementation aspects including physical design, signal and power integrity, and design for testability, are also discussed.

Terahertz Technologies to Deliver Optical Network Quality of Experience in Wireless Systems Beyond 5G

“…GHz [5], [10]. Although the upper-bound calculation is based for simplicity on ideal transmitter and receivers without additional impairments like I/Q imbalances or phase noise, this is justified by assuming the use of impairments mitigating DSP, which can be effectively used to idealize optical transceivers [11], [12]. The uncorrected amplitude and phase mismatches are in the order GHz, links are dominantly attenuated by the FSPL, whereas in the THz regime, the molecular absorption becomes more important, due to its exponentially increasing impact as a function of distance.…”

“…For all transistor technology options, direct conversion transceiver architectures are currently DRAFT March 28, 2018 the most widespread, due to their low integration complexity. However, when targeting the use of higher-order modulations, those architectures suffer from imperfections that need to be mitigated by baseband DSP [3], [11], [12]. Most critical is the correction of IQ imbalances, amplifier nonlinearities and local oscillator (LO) phase noise (PHN).…”

“…The error-correction schemes and frontend requirements in photonic radio systems are a subject for research. In recent years, there has been great progress reported in developing mitigation schemes for carrier PHN, nonlinear dispersion, and IQ imbalance for coherent fiber optical systems enabling 64-QAM in future systems [12]. The baseband interface plays an important role, since it needs to handle symbol rates of up to 64 Gbd, support at least 64-QAM, map the symbols to the optical-fiber electronics and take care of pre-equalization.…”

Modulation Format Independent Joint Polarization and Phase Tracking for Coherent Receivers

“…The noise term is s n k = (H k u k ) H σ σ σn k + n H k σ σ σH k u k + n H k σ σ σn k and can be identified by equating terms after substituting (13) in s r k = r H k σ σ σr k . As can be noted, s n k is signal dependent and (31) is not an additive noise model, opposed to (13). The channel matrix M k modeling the evolution of the SOP can be expressed using a 3 × 3 Mueller matrix defined as [18]…”

Improved Low-Power LDPC FEC for Coherent Optical Systems

“…The applicationspecific integrated circuit (ASIC) implementations of these high-throughput, computationally complex decoders also require high silicon area, which translates directly to high capital cost. Careful consideration of the trade-offs between conflicting performance and complexity requirements is therefore needed when designing such FEC systems 6 .…”